Stability investigation of the mixed DPPC/protein monolayer at the air-water interface

A subphase exchange method has been used to investigate the stability of th e mixed L-alpha-dipalmitoylphosphatidylcholine (DPPC)/beta-lactoglobulin mo nolayer formed via the protein adsorption into the spread lipid layer at th e air-water interface by Brewster angle microscopy (BAM). The BAM images de monstrated that the adsorbed protein might compress the DPPC monolayer to e nter a phase transition early close to a condensed phase. When protein conc entration is very high the penetration of the protein is rather fast, and a larger domain was preferentially formed without compressing the DPPC monol ayer. After the mixed DPPC/beta-lactoglobulin monolayer was formed, the pro tein solution in subphase was totally washed out by pumping in buffer water for a long time. In the fluid phase and coexistence region of DPPC monolay er with penetrated protein layer the domains have been recorded by BAM imag es for comparing the variation before and after the exchange of subphase. T he experimental results revealed that there was not significant change of t he domains in size and shape. It indicates that the adsorbed protein has a strong interaction with DPPC and is more likely to remain at the interface. A relatively stable mixed lipid-protein monolayer can be constructed by th is way. Hydrophobic and electrostatic interactions between DPPC and beta-la ctoglobulin as well as the conformation change of beta-lactoglobulin at air -water interface have been taken into account to stabilize the mixed layers , (C) 2000 published by Elsevier Science B.V. All rights reserved.